Urea synthesis process and apparatus

ABSTRACT

Urea is prepared by reacting ammonia and carbon dioxide in an apparatus comprising a vertical condensation and synthesis column and a stripper, to provide a urea synthesis solution comprising urea, unreacted ammonia, unreacted carbon dioxide and water. The urea synthesis solution is transferred from the top of the vertical condensation and synthesis column to the top of a stripper. Carbon dioxide is introduced into the bottom of the stripper and contacted with the urea synthesis solution, thereby separating the unreacted ammonia and the unreacted carbon dioxide from the urea, and providing a mixed gas comprising ammonia, carbon dioxide and water. The mixed gas is transferred into the bottom of the vertical condensation and synthesis column, where it is reacted with liquid ammonia injected into the bottom and a middle of the vertical condensation and synthesis column. The mixed gas and liquid ammonia are condensed and react to form urea. Uncondensed gases are absorbed in an absorbing medium, which is subsequently recycled to the bottom of the vertical condensation and synthesis column.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a urea synthesis methodcomprising separating unreacted ammonia and unreacted carbon dioxidefrom the reaction mixture by stripping the mixture with the entirecarbon dioxide feedstock, at a pressure substantially equal to the ureasynthesis pressure.

[0003] 2. Description of the Related Art

[0004] Methods for preparing urea in which ammonia is reacted withcarbon dioxide, are well known, and a variety of proposals have beenmade for urea synthesis methods. For example, ammonia may be reactedwith carbon dioxide in a urea synthesis zone at a desired urea synthesispressure and urea synthesis temperature, and the unreacted ammoniumcarbamate may then be separated from the product urea synthesis solutionas a mixed gas of ammonia and carbon dioxide. This mixed gas may then beabsorbed in an absorbing medium and recirculated to the urea synthesiszone, thereby providing a urea solution from which the ammoniumcarbamate has been separated from the urea synthesis solution.

[0005] Generally, a urea plant consists of a synthesis column, acarbamate condenser and a stripper. Often, the bottoms of the synthesiscolumn and the carbamate condenser are placed at a height of 20 to 30meters above the ground. In this case, the supporting structure iscomposed of concrete was laid up to this height, or a combination ofconcrete with a steel framed structure was built, rather than solely asteel-frame structure. In light of the time and effort required for theinstallation and maintenance of such apparati, urea synthesis processeswith urea synthesis columns and the like placed on the ground have beendeveloped.

[0006] The present inventors have previously proposed (Japanese PatentLaid-Open No. 10-182587) an improved method of synthesizing urea,comprising stripping unreacted ammonia and carbon dioxide from theproduct mixture with the carbon dioxide feedstock. This method allowsthe equipment to be placed on the ground, rather than elevated asdescribed above. This method comprises providing a vertical condenserabove the urea synthesis column so that the mixed gas from the strippercontacts the absorbing medium under cooled conditions, in order tocondense the mixed gas. The apparatus in which this method is practicedhas a first down-flow-pipe passing from the top of the condenser to thebottom of the synthesis column in order to allow the resultingcondensate to flow down to the bottom of the synthesis column by meansof gravity. This condensate liquid, along with a part of the liquidammonia or carbon dioxide starting materials participate in thesynthesis of urea. The urea synthesis solution, provided by means of asecond down-flow-pipe having an opening in the top of the synthesiscolumn, is introduced into the stripper by means of gravity. Theunreacted ammonia and carbon dioxide, i.e., the above-described mixedgas, is separated with the remainder of the carbon dioxide startingmaterial and introduced into the bottom of the above-described condenserand a condensed, or the condensate liquid is sucked from the verticalcondenser into the bottom of the urea synthesis by means of an ejectorusing preheated liquid ammonia starting material as a driving fluid topromote the synthesis of urea from the condensate.

[0007] However, in the above-described method, the ammonia and carbondioxide separated at the top of the condenser are absorbed by theabsorbing medium in the scrubber, without being recycled to the bottomof the urea synthesis column, and thus do not add heat to the ureasynthesis column (i.e., because the heat of formation of ammoniumcarbamate due to the reaction between ammonia and carbon dioxide is notgenerated in the urea synthesis column). Thus, it is desirable toproduce ammonium carbamate in the urea synthesis column by introducing aportion of the carbon dioxide feedstock into the urea synthesis column,in order to raise its temperature by means of the heat of the reaction.

[0008] Stamicarbon B. V. has also proposed a method in which thesynthesis of urea and absorption of uncondensed gas is performed in onehorizontal apparatus using a horizontal condenser combined with ahorizontal urea synthesis reactor. The mixed gas containing ammonia andcarbon dioxide from the stripper may be condensed to produce ammoniumcarbamate, in order to recover the heat formed at this time.

[0009] However, in this method, since the liquid flows only in thehorizontal direction and the gas flows only upward, the liquid-gascontact is insufficient and the condensation rate of the gas remainslow. Also, an almost ideal plug flow cannot be obtained, and thereforethe rate of urea synthesis is low, thereby increasing the requiredcapacity of this horizontal apparatus per unit production. Also, a largesurface area is required for installing a horizontal apparatus.Furthermore, in order for the gas to flow uniformly in a horizontallyoriented apparatus, a gas dispersion unit with a large pressure loss isrequired. There are also various additional problems, such as the largespace required for liquid-gas separation.

SUMMARY OF THE INVENTION

[0010] One object of the present invention is to provide a ureasynthesis process in which a mixed gas of unreacted ammonia and carbondioxide has sufficient contact with a liquid phase, the requiredcapacity of an apparatus employing this method, per unit production, issmall, and condensation of the mixed gas and synthesis of urea areunified. This is provided by reacting ammonia and carbon dioxide in avertical condensation and synthesis zone, stripping the resulting ureasynthesis solution with the carbon dioxide feedstock in a strippingzone, and recycling the unreacted carbon dioxide and ammonia obtained bystripping the urea synthesis solution back to the vertical condensationand synthesis zone.

[0011] A second object of the present invention is to provide a ureasynthesis apparatus for carrying out the above described process.

[0012] Other objects of the present invention will be apparent from thefollowing descriptions.

[0013] The above described objects of the present invention are achievedby the urea synthesis process and apparatus described below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a diagram illustrating a first embodiment of the presentinvention.

[0015]FIG. 2 is a diagram illustrating a second embodiment of thepresent invention, in which the bottom of a vertical condensation andsynthesis column and the bottom of a stripper are both placed atsubstantially the same level (e.g., on the ground).

[0016]FIG. 3 is a diagram showing one example of a baffle plate for usein the present invention.

[0017]FIG. 4 is a diagram illustrating the process of ComparativeExample 1.

[0018]FIG. 5 is a diagram illustrating the process of ComparativeExample 2.

DETAILED DESCRIPTION OF THE INVENTION

[0019] A first embodiment of the present invention is a urea synthesisprocess in which ammonia is reacted with carbon dioxide in a verticalcondensation and synthesis zone at a temperature and pressure sufficientto provide a urea synthesis solution comprising urea, unreacted ammonia,unreacted carbon dioxide and water. The urea synthesis solution is thencontacted with the carbon dioxide feedstock in a heated stripping zoneat a pressure substantially equal to the urea synthesis pressure,thereby separating the unreacted ammonia and the unreacted carbondioxide as a mixed gas of ammonia, carbon dioxide and water. The ureasynthesis solution, containing residual unreacted ammonia and unreactedcarbon dioxide that were not separated in the stripping zone, is furtherprocessed to provide urea, while the mixed gas separated in thestripping zone is introduced into the condensation and synthesis zoneand contacted with an absorbing medium and liquid ammonia and cooled tocondense the mixed gas and carry out the synthesis of urea. The mixedgas and the absorbing medium are supplied to a bottom part of thevertical condensation and synthesis zone, the liquid ammonia feedstockis supplied to the bottom and middle part of the vertical condensationand synthesis zone, and the urea synthesis solution flows down, by meansof gravity, from a top portion of the vertical condensation andsynthesis zone to a top portion of the stripping zone.

[0020] The pressure at which the synthesis of urea is carried out in thevertical condensation and synthesis zone is preferably in the range of13 to 25 MPaG (gauge pressure), including pressures of 14, 15, 16, 17,18, 19, 20, 21, 22, 23, and 24 MPaG, inclusive of all values andsubranges therebetween. In the condensation portion of the condensationand synthesis zone, the temperature of the liquid is preferably kept at165 to 190° C., including 170, 180, and 185° C. The mole ratio ofammonia to carbon dioxide (abbreviated as N/C) and the mole ratio ofwater to carbon dioxide (abbreviated as H/C) in the condensation portionof the condensation and synthesis column are preferably kept at 2.5 to4.5, and 1.0 or smaller, respectively. The temperature of the liquid inthe synthesis portion of the condensation and synthesis zone ispreferably kept at 170 to 200° C., including 175, 180, 185, 190 and 195°C., inclusive of all values and subranges therebetween. The N/C and H/Cratios in the synthesis portion of the condensation and synthesis zoneare preferably 3.5 to 5.0, and 1.0 or smaller, respectively.

[0021] In the vertical condensation and synthesis zone, a percent ureaconversion of 20 to 75%, preferably 60 to 75%, may be achieved.

[0022] The pressure of the stripping zone is substantially equal to thatof the condensation and synthesis zone, i.e., 13 to 25 MPaG (gaugepressure), including pressures of 14, 15, 16, 17, 18, 19, 20, 21, 22,23, and 24 MPaG, inclusive of all values and subranges therebetween. Theterm “substantially equal” means that the pressures of the strippingzone and the vertical condensation and synthesis zone differ by 0.5 MPaor less. The temperature of the stripping zone is preferably kept at 160to 200° C., including 165, 170, 175, 180, 185, 190, and 195° C.,inclusive of all values and subranges therebetween. Preferably, in orderto increase the rate of stripping, unreacted ammonia and carbon dioxidefrom the urea synthesis solution, the full amount of the carbon dioxidestarting material is used for stripping.

[0023] A second embodiment of the present invention is a urea synthesisprocess in which ammonia is reacted with carbon dioxide in a verticalcondensation and synthesis zone at a temperature and pressure sufficientto provide a urea synthesis solution comprising urea, unreacted ammonia,unreacted carbon dioxide and water. The urea synthesis solution is thencontacted with the carbon dioxide feedstock in a heated stripping zoneat a pressure equal to or slightly lower (i.e., at a pressure 0.05 to 5MPa lower) than the urea synthesis pressure, thereby separating theunreacted ammonia and the unreacted carbon dioxide as a mixed gas ofammonia, carbon dioxide and water. The urea synthesis solution,containing residual unreacted ammonia and unreacted carbon dioxide thatwere not separated in the stripping zone, is further processed toprovide urea, while the mixed gas separated in the stripping zone isincreased in pressure 0.05 to 5 MPa and introduced into the bottomportion of the condensation and synthesis zone, contacted with anabsorbing medium and liquid ammonia and cooled to condense the mixed gasand carry out the synthesis of urea. The absorbing medium is supplied toa bottom part of the vertical condensation and synthesis zone and theliquid ammonia feedstock is supplied to the bottom and middle part ofthe vertical condensation and synthesis zone.

[0024] The condensation and synthesis zone of the process of the secondembodiment is maintained at a pressure, temperature, N/C and H/C ratioas described above for the process of the first embodiment of thepresent invention. However, in the process of the second embodiment, thestripping zone is operated at a pressure equal to or slightly lower(i.e., 0.05 to 5 MPa lower) than the pressure of the condensation andstripping zone. The temperature and N/C and H/C ratios of the process ofthe third embodiment are as described above for the process of the firstembodiment.

[0025] A third embodiment of the present invention is a urea synthesisprocess in which ammonia is reacted with carbon dioxide in a verticalcondensation and synthesis zone at a temperature and pressure sufficientto provide a urea synthesis solution comprising urea, unreacted ammonia,unreacted carbon dioxide and water. The urea synthesis solution is thencontacted with carbon dioxide in a heated stripping zone at a pressuresubstantially equal to the urea synthesis pressure, thereby separatingthe unreacted ammonia and the unreacted carbon dioxide as a mixed gas ofammonia, carbon dioxide and water. The urea synthesis solution,containing residual unreacted ammonia and unreacted carbon dioxide thatwere not separated in the stripping zone, is further processed toprovide urea, while the mixed gas separated in the stripping zone isincreased in pressure 0.05 to 5 MPa and introduced into the bottomportion of the condensation and synthesis zone, contacted with anabsorbing medium and liquid ammonia and cooled to condense the mixed gasand carry out the synthesis of urea. The mixed gas and the absorbingmedium are supplied to a bottom part of the vertical condensation andsynthesis zone, the liquid ammonia is supplied to the bottom and middlepart of the vertical condensation and synthesis zone, and the ureasynthesis solution is removed from a top portion of the condensation andsynthesis zone and introduced to a top portion of the stripping zone.

[0026] The condensation and synthesis zone and stripping zone of theprocess of the third embodiment are maintained at a pressure,temperature, N/C and H/C ratio as described above for the process of thefirst embodiment of the present invention.

[0027] A fourth embodiment of the present invention is an apparatuscomprising (a) a vertical condensation and synthesis column having acooler in a portion extending from a bottom portion (i.e., the bottomportion is approximately the lower third of the condensation andsynthesis column, preferably the lower 20% of the column) to a middleportion (i.e., the middle portion is approximately the central third ofthe condensation and synthesis column, preferable the central 20% of thecolumn), (b) a stripper for separating a mixed gas of unreacted ammonia,unreacted carbon dioxide and water from a urea synthesis solution bystripping the urea synthesis solution with carbon dioxide, (c) a pipefor supplying liquid ammonia to the bottom portion and middle portion ofthe vertical condensation and synthesis column, (d) a pipe that has anopening at one end thereof in a top portion of the vertical condensationand synthesis column (i.e., the top portion is approximately the topthird of the condensation and synthesis column, preferably the top 20%of the column) connected to a top portion of the stripper (i.e.,similarly, the top portion of the stripper is approximately the topthird of the stripper, preferably the top 10% of the stripper), forintroducing the urea synthesis solution into a top portion of thestripper by means of gravity, (e) a pipe connected to the bottom portionof the vertical condensation and synthesis column for introducing themixed gas from the stripper to the vertical condensation and synthesiscolumn, (f) a scrubber provided in the top portion of the verticalcondensation and synthesis column for absorbing, with an absorbingmedium, the ammonia and carbon dioxide from an inert gas separated fromthe urea synthesis solution in the vertical condensation and synthesiscolumn, (g) a pipe for discharging the inert gas from the top portion ofthe scrubber, (h) a pipe for introducing into the bottom portion of thevertical condensation and synthesis column the absorbing mediumcontaining absorbed ammonia and carbon dioxide from the scrubber, (i) apipe for introducing carbon dioxide into the bottom portion of thestripper, and (j) a pipe for discharging an aqueous urea solutioncontaining residual unreacted ammonia and unreacted carbon dioxide thatare not separated from the aqueous urea solution from the bottom portionof the stripper (i.e, the bottom portion of the stripper isapproximately the bottom third of the stripper, preferably the bottom10% of the stripper).

[0028] In the apparatus of the fourth embodiment of the presentinvention, a portion of the ammonia feedstock, the absorbing medium andthe mixed gas from the stripper are introduced into the bottom portionof the vertical condensation and synthesis column, and the remainder ofthe liquid ammonia feedstock is introduced in the middle portion thecondensation and synthesis column. A collection of U-pipes are providedfor the portion of the vertical condensation and synthesis columnextending from the bottom of the column to the middle portion thereof,and water, for example, flows through this collection of U-pipes to coolthe lower portion of the condensation and synthesis column. This portionof the condensation and synthesis column is a condensing portion whichfunctions similarly to the condenser in a conventional urea synthesisapparatus.

[0029] The portion above the middle portion of the vertical condensationand synthesis column (i.e., the portion above the condensation portionof the column) is the synthesis portion of the column, and performs thefunction of a urea synthesis column. Condensed liquid containingammonium carbamate, produced in the condensation portion of the column,rises to the synthesis portion, along with a small amount of uncondensedgas. Liquid ammonia is supplied to the synthesis portion, and is reactedwith the carbon dioxide in the uncondensed gas to produce the ammoniumcarbamate.

[0030] The top part of the synthesis portion is provided with an openingfor a pipe to deliver the urea synthesis solution produced in thesynthesis portion of the column to the top part of the stripper. Theportion above the liquid surface of the top part of the synthesisportion is a liquid-gas separation portion for separating theuncondensed gas from the urea synthesis solution. A scrubbing portion isprovided above the liquid-gas separation portion to wash and absorbseparated gas with the absorbing medium. The absorbing medium is aliquid which contains ammonium carbamate obtained by the reaction ofpreviously unreacted ammonia and carbon dioxide contained in the ureasolution discharged from the stripper, and an ammonium carbamatesolution that is obtained by collecting ammonia and carbon dioxidedischarged along with an inert gas from an absorbing portion.

[0031] The absorbing medium that has absorbed ammonia and carbon dioxidein the scrubbing portion is allowed to flow down through the pipe intothe bottom part of the vertical condensation and synthesis column.

[0032] The ratios of volumes of the condensing portion of the column,the synthesis portion of the column, and the separation portion andscrubbing portion of the column to the whole volume of the verticalcondensation and synthesis column are preferably in the range of 40 to50%, 40 to 50%, 3.0 to 7.0% and 1.0 to 5.0%, respectively. Preferably,1.0 to 50.0% of liquid ammonia is introduced into the middle part of thevertical condensation and synthesis column.

[0033] As discussed above, when the processes of the present inventionare carried out in the apparatus of the fourth embodiment of the presentinvention, the pressure of the vertical condensation and synthesiscolumn is preferably in the range of 13 to 25 MPaG (and values andsubranges therebetween, as discussed above). Likewise, the temperature,N/C and H/C of the liquid in the condensation portion of the column arepreferably maintained at 165 to 190° C., 2.5 to 4.5 and 1.0 or smaller,respectively (and values and subranges therebetween, as discussedabove). The temperature, the N/C and the H/C of the liquid in thesynthesis portion of the column are preferably maintained at 170 to 200°C., 3.5 to 5.0 and 1.0 or smaller, respectively (and values andsubranges therebetween, as discussed above).

[0034] In the vertical condensation and synthesis column of the presentinvention, urea yields of up to 60 to 75% may be achieved.

[0035] It is preferable that in the vertical condensation and synthesiscolumn, at least the condensation portion is provided with a baffleplate to improve liquid-gas contact.

[0036] Stripping is performed while the pressure of the stripper is keptat a pressure substantially equal to or slightly lower than the ureasynthesis pressure (depending upon the process conditions employed) andthe temperature of the stripper is preferably kept at 160 to 200° C. Thefull amount of carbon dioxide is preferably used for stripping in orderto increase the rates of stripping of unreacted ammonia and carbondioxide from the urea synthesis solution.

[0037] In the apparatus of the fourth embodiment, the urea synthesissolution obtained from the top of the vertical condensation andsynthesis column is allowed to flow down to the top of the stripper bymeans of gravity.

[0038] A fifth embodiment of the present invention is an apparatuscomprising (a) a vertical condensation and synthesis column placed onthe ground, having a cooler in a portion extending from a bottom portionto a middle portion thereof, (b) a stripper placed at substantially thesame level, (i.e., the vertical elevation of the bottom of the stripperand the column differ by no more than 1 meter, e.g., placed on theground) for separating a mixed gas of unreacted ammonia, unreactedcarbon dioxide and water from a urea synthesis solution by stripping theurea synthesis solution with carbon dioxide, (c) a pipe for supplyingliquid ammonia to the bottom portion and middle portion of the verticalcondensation and synthesis column, (d) a pipe comprising pressureraising means (for example, a pump or compressor located approximatelyin the middle portion of the pipe), which has an opening in an endthereof in a top portion of the vertical condensation and synthesiscolumn for introducing the urea synthesis solution into a top portion ofthe stripper, (e) a pipe for introducing into the bottom portion of thevertical condensation and synthesis column the mixed gas from thestripper, (f) a scrubber that is provided in the top portion of thevertical condensation and synthesis column for absorbing, with anabsorbing medium, ammonia and carbon dioxide in an inert gas separatedfrom the urea synthesis solution in the vertical condensation andsynthesis column, (g) a pipe for discharging the inert gas from a topportion of the scrubber, (h) a pipe for introducing the absorbing mediumcontaining absorbed ammonia and carbon dioxide from the scrubber intothe bottom portion of the vertical condensation and synthesis column,(i) a pipe for introducing carbon dioxide into a bottom portion of thestripper, and (j) a pipe for discharging an aqueous urea solutioncontaining residual unreacted ammonia and unreacted carbon dioxide thatare not separated from the aqueous urea solution, from the bottomportion of the stripper.

[0039] The apparatus of the fifth embodiment may be operated similarlyto the apparatus of the fourth embodiment, using the pressuretemperature, N/C, and H/C ratios as described above. However, in thefifth embodiment, the pipe (d) is provided with pressure raising means(i.e., a compressor or pump) which raises the pressure of the ureasynthesis solution introduced into the top part of the stripper by 0.05to 0.5 MPa.

[0040] A sixth embodiment of the present invention is an apparatuscomprising (a) a vertical condensation and synthesis column placed onthe ground, having a cooler in a portion extending from a bottom portionto a middle portion, (b) a stripper placed at substantially the samelevel for separating a mixed gas of unreacted ammonia, unreacted carbondioxide and water from a urea synthesis solution by stripping the ureasynthesis solution with the carbon dioxide feedstock, (c) a pipe thathas an opening in one end in a top portion of the vertical condensationand synthesis column, connected to the stripper, for introducing a ureasynthesis solution into the top portion of the stripper, (d) a pipe forsupplying liquid ammonia feedstock to the bottom portion and the middleportion of the vertical condensation and synthesis column, (e) a pipecomprising pressure raising means (for example, in the middle portion ofthe pipe), for introducing the mixed gas from the stripper into thebottom portion of the vertical condensation and synthesis column, (f) ascrubber that is provided in the top portion of the verticalcondensation and synthesis column for absorbing, with an absorbingmedium, the ammonia and carbon dioxide in the inert gas separated fromthe urea synthesis solution in the vertical condensation and synthesiscolumn, (g) a pipe for discharging the inert gas from a top portion ofthe scrubber, (h) a pipe for introducing the absorbing medium from thescrubber containing absorbed ammonia and carbon dioxide into the bottomportion of the vertical condensation and synthesis column, (i) a pipefor introducing the carbon dioxide feedstock into a bottom portion ofthe stripper, and (j) a pipe for discharging an aqueous urea solutioncontaining residual unreacted ammonia and unreacted carbon dioxide thatare not separated from the aqueous urea solution from the bottom portionof the stripper.

[0041] The apparatus of the sixth embodiment may be operated similarlyto the apparati of the fourth and fifth embodiments, using the pressuretemperature, N/C, and H/C ratios as described above. However, in thesixth embodiment a pipe (e) is provided with pressure raising means(e.g., a blower) to raise the pressure of the mixed gas from thestripper, which is introduced in the bottom portion of the verticalcondensation and synthesis column, by 0.05 to 5 MPa.

[0042] In the processes and apparati of present invention, it is notnecessary to supply part of the carbon dioxide feedstock to thesynthesis column in order to maintain the temperature of the synthesiscolumn by means of the heat of reaction of carbon dioxide with ammonia,because a part of the liquid ammonia feedstock, a mixed gas of ammoniaand carbon dioxide from the stripper, and the absorbing medium aresupplied to the bottom part of the vertical condensation and synthesiscolumn. Thus, an amount of ammonia and carbon dioxide equivalent to theamount discharged from the top of the condenser are effectively added tothe vertical condensation and synthesis column as in cases where thecondenser is separated from the synthesis column. Since the full amountof the carbon dioxide feedstock can be used for stripping, the rate ofstripping is increased, and the recovery rate required in the downstream part of the process is reduced.

[0043] Also, by using a vertical condensation and synthesis column, thecapacity coefficient of the liquid phase (K_(L)a, the rate at which thegas is dissolved in the liquid) is twice as large as the case in which ahorizontal condensation and synthesis column is used, and thus the rateof urea synthesis is increased. In addition, the burden on the stripperis reduced.

[0044] The present invention will be further described below, referringto the Figures. FIG. 1 is a diagram which illustrates the fourthembodiment of the present invention. In FIG. 1, the liquid ammoniafeedstock is supplied from a pipe 2 to the bottom portion and the middleportion of a vertical condensation and synthesis column 1 through pipes3 and 4, respectively. A gas mixture comprising ammonia, carbon dioxideand water which is separated in a stripper 21 and an absorbing mediumthat will be described later are introduced into the bottom portion ofthe vertical condensation and synthesis column 1 through pipe 22 andpipe 5, respectively.

[0045] The condensation portion A (the portion extending from the bottomto the middle portion) of the vertical condensation and synthesis column1 is provided with cooling pipes 6. The cooling pipes preferablycomprise a collection of inverse U-pipes. Water (or any other suitableheat exchange medium) is passed through the cooling pipes as a coolant,and the heat of formation from ammonium carbamate produced in a coolingportion A can be collected by converting the water in the U-pipes tosteam. The ammonium carbamate produced therein rises through thecondensation portion A into a synthesis portion B (the portion extendingfrom the middle portion to the liquid surface of the top portion) of thevertical condensation and synthesis column 1, along with uncondensedammonia and carbon dioxide. The operating conditions in the condensingportion A, i.e., the pressure, the N/C, the H/C, the temperature and theresidence time are preferably 13 to 25 MPaG, 2.5 to 4.5, 1.0 or smaller,165 to 190° C. and 10 to 30 minutes, respectively. Under theseconditions, a urea yield of 20 to 60% may be achieved. The condensationrate will decrease and the temperature at which the ammonium carbamateis solidified will increase due to an increase in the partial pressureof carbon dioxide in the ammonium carbamate solution if the N/C issmaller than 2.5, and the rate of condensation will decrease due to arise in the vapor pressure of ammonia if the N/C is greater than 4.5.Therefore, N/C is preferably 2.5 to 4.5. Also, if the H/C is greaterthan 1.0, the rate of urea synthesis at equilibrium will decrease.Accordingly, the H/C is preferably 1.0 or smaller. If the residence timeis shorter than 10 minutes, the vapor pressure will rise and the rate ofcondensation will decrease due to a drop in the rate of urea synthesis.If the residence time is longer than thirty minutes, no significantincrease in the rate of urea synthesis may be expected, therebyrequiring an extreme increase in the capacity required in the condensingportion A. Thus, the residence time is preferably 10 to 30 minutes. Therate of urea synthesis decreases if the operation temperature issignificantly lower than 170° C., and if the operation temperature issignificantly higher than 190° C. the rate of condensation decreases andthe corrosion of the materials of apparatus may increase due to a risein vapor pressure. Accordingly, the operating temperature is preferably170 to 190° C.

[0046] The condensed liquid introduced in the synthesis portion B fromthe condensation portion A of the vertical condensation and synthesiscolumn 1 is mixed with the liquid ammonia feedstock introduced throughpipe 4, thereby producing ammonium carbamate from the uncondensedammonia and carbon dioxide. The heat of formation of the ammoniumcarbamate is used to provide the heat of reaction for the endothermicreaction through which ammonium carbamate is converted to urea.Furthermore, the temperature, the N/C, the H/C and residence time in thesynthesis portion B are preferably 180 to 200° C., 3.5 to 5.0, 1.0 orsmaller and 10 to 40 minutes, respectively, which provides a urea yieldof 60 to 75%. If the N/C is smaller than 3.5, the equilibrium rate ofsynthesis will decrease. If the N/C exceeds 5.0, the ammonia will begasified, resulting in an increase in the vapor pressure. If the H/C isgreater than 1.0, the rate of synthesis will decrease. If the residencetime of the reactants in synthesis portion B is shorter than tenminutes, the maximum rate of synthesis cannot be achieved, and if theresidence time is greater than forty minutes, the maximum rate ofsynthesis has already been attained and further increases in the rate ofsynthesis would not be expected.

[0047] In the top portion of the synthesis portion B of the verticalcondensation and synthesis column 1, the upper end of pipe 7 is providedin the vertical condensation and synthesis column in order to allow theurea synthesis solution produced in the condensation and synthesiscolumn to flow into a stripper 21. A liquid surface is formed above theopening of pipe 7. The portion above the liquid surface is a liquid-gasseparation portion C, which performs the function of separating inertgases (hydrogen and nitrogen contained in the carbon dioxide as well asair introduced in order to prevent the corrosion of apparatus) andaccompanying ammonia and carbon dioxide from the urea synthesissolution.

[0048] The topmost portion of the vertical condensation and synthesiscolumn 1 is provided with a scrubbing portion D having the function ofwashing the gas separated in the liquid-gas separation portion C byabsorbing by the ammonia and carbon dioxide in the gas with an absorbingmedium. The absorbing medium may be a liquid containing the ammoniumcarbamate obtained by collecting the unreacted ammonia and carbondioxide in the urea solution from the stripper, which is introduced intothe top part of the scrubbing portion D by a pipe 8, as described below.The introduced absorbing medium contacts the gas rising from theliquid-gas separation portion C in a packed bed 9, absorbs a part ofammonia and carbon dioxide therein, and flows down to the bottom of thevertical condensation and synthesis column 1 through pipe 5, which hasan opening in a chimney portion 10. The pipe 5 is preferably providedinside the vertical condensation and synthesis column 1, but may also beprovided outside the vertical condensation and synthesis column 1.

[0049] Preferably, at least the condensation portion A of the verticalcondensation and synthesis column 1 may be provided with a baffle plateto improve liquid-gas contact. A variety of known baffle plates may beused. One example thereof is a type of baffle plate designed to makegases and liquids flow separately, as shown in FIG. 3. In FIG. 3, thegas flows upward through a plurality of holes 31 disposed betweencooling pipes 6 and is cooled, while the liquid flows upward in a zigzagpath (i.e., by “zigzag path” we mean a non-linear path) through a liquidpassage 33 provided in the periphery of the baffle plate 30, in such amanner that the liquid intersects the flow of the gas. (The liquidpassage of the upper or lower baffle plate is provided in a symmetricalposition). Furthermore, an optional convex part 32 is preferablyprovided in the central part of the baffle plate in order to gather thegas in the plurality of small holes 31.

[0050] The unabsorbed gas containing ammonia and carbon dioxide that isdischarged from a line 15 in the top of the scrubbing portion D (FIG. 1)is washed by water in order to collect the ammonia and carbon dioxide.

[0051] The urea synthesis solution flowing down through pipe 7 from thetop portion of the synthesis portion B of the vertical condensation andsynthesis column 1 is introduced into the top portion of the stripper 21by means of a pipe 11 and a controlling valve 12. The urea synthesissolution introduced through pipe 11 and valve 12 flows down through aheater in the stripper 21 (steam is introduced from a line 23 andcondensate is discharged from a line 24) and contacts the carbon dioxideintroduced into the bottom portion of the stripper 21 through pipe 13,thereby separating most of the unreacted ammonia and carbon dioxide fromthe urea synthesis solution. The separated mixture of unreacted ammoniaand unreacted carbon dioxide, together with the carbon dioxide addedthrough pipe 13, is supplied to the vertical condensation and synthesiscolumn 1 through a pipe 22.

[0052] Particularly preferably, the operating pressure and thetemperature of stripper 21, are 14 to 20 MPaG and 160 to 200° C.,respectively. The total amount of carbon dioxide used in the synthesisof urea is preferably supplied to the stripper 21. Although not shown inFIG. 1, a portion of the carbon dioxide may be supplied to a lowpressure decomposer for the remaining ammonium carbamate.

[0053] The liquid containing urea and the unreacted ammonia and carbondioxide is withdrawn from the bottom part of the stripper, andtransferred through pipe 25 to a low pressure decomposition step, andprocessed to provide urea. The unreacted ammonia and carbon dioxide areabsorbed by water, dilute aqueous ammonia or the like, to provide arecovered solution. This recovered solution may be supplied as anabsorbing medium to the scrubbing portion D of the vertical condensationand synthesis column 1, as described above.

[0054]FIG. 2 is a diagram which illustrates another embodiment of theapparatus of the present invention, in which the bottom of both thevertical condensation and synthesis column 101 and stripper 121 areplaced at substantially the same vertical elevation (e.g., on theground). The apparatus described in FIG. 2 differs from that of FIG. 1only in that a blower 126 for raising the pressure of the mixed gas by0.05 to 5 MPa is provided at some point (e.g., the midpoint) in pipe 122used to transfer the mixed gas from the stripper to the verticalcondensation and synthesis column 101, or the line 111 for transferringthe urea synthesis solution from vertical condensation and synthesiscolumn 101 to the stripper is provided with a pump 114 for raising thepressure of the urea synthesis solution by 0.05 to 0.5 MPa. Furthermore,the blower 126 and the pump 114 are never used together. Thus, if theapparatus of FIG. 2 has the blower 126, it does not have the pump 114,and if the pump 114 is present, the apparatus of FIG. 2 does not havethe blower 126.

[0055] Having generally described this invention, a furtherunderstanding can be obtained by reference to certain specific exampleswhich are provided below for purposes of illustration only, and are notintended to be limiting unless otherwise specified.

EXAMPLE 1

[0056] An example of the production of 1725 tons of urea per day usingthe apparatus of FIG. 1 is presented below.

[0057] Liquid ammonia feedstock preheated to 75° C. by a pre-heater 20was divided into two streams through pipe 2, so that 25.48 tons per dayof liquid ammonia feedstock were introduced via pipe 3 into the bottompart of the vertical condensation and synthesis column 1, and 950.13tons per day were introduced via pipe 4 into the middle part of thevertical condensation and synthesis column 1.

[0058] A mixed gas of 1608.32 tons per day of ammonia, 1637.91 tons perday of carbon dioxide and 150.57 tons per day of water from the top ofthe stripper 21 was introduced into the bottom part of the verticalcondensation and synthesis column 1 via pipe 22. The recovered solutioncomprised 5.17 tons per day of urea. A mixture of 389.68 tons per day ofammonia, 482.63 tons per day of carbon dioxide and 320.42 tons of water,obtained from the stripper by separating and recovering the unreactedammonia and carbon dioxide from the urea solution, was supplied to thetop part of the scrubbing portion D via pipe 8, in order to absorb theammonia and carbon dioxide from the uncondensed gas. The resultingabsorbing liquid flowed down into the bottom of the verticalcondensation and synthesis column 1 through pipe 5.

[0059] The vertical condensation and synthesis column 1 was maintainedat a pressure of 15 MPa, and the temperature of the condensation portionA was maintained at 180° C. by cooling the condensation portion A withcooling pipes 6, thereby producing ammonium carbamate from ammonia andcarbon dioxide. The condensed liquid containing ammonium carbamateflowed upward to the synthesis portion 8 along with the uncondensedammonia and carbon dioxide, and the mixture was heated by the reactionof the liquid ammonia introduced via pipe 4, with the uncondensedammonia and the uncondensed carbon dioxide, thereby providing urea. Theresidence time in the vertical condensation and synthesis column wasforty minutes.

[0060] The urea synthesis solution product was introduced into the toppart of the stripper 21 through pipes 7 and 11 and control valve 12. Theremaining uncondensed ammonia and carbon dioxide was separated in theliquid-gas separation portion C. The composition of the urea synthesissolution comprised 1853.90 tons per day of urea, 1881.69 tons per day ofammonia, 744.74 tons per day of carbon dioxide, 1023.41 tons per day ofwater and 4.31 tons of biuret.

[0061] 1211.59 tons of the carbon dioxide feedstock were supplied viapipe 13 to the stripper 21 and was contacted with the urea synthesissolution flowing down from the top part of the stripper at a pressure of15 MPaG, at a temperature of 185° C., thereby removing the unreactedammonia and carbon dioxide in the urea synthesis solution, to providethe aforesaid mixed gas.

[0062] The gas containing ammonia and carbon dioxide separated in theliquid-gas separation portion C was absorbed by the absorbing medium,comprising aqueous ammonium carbamate obtained by separating/recoveringthe unreacted ammonia and carbon dioxide in the urea solution from thestripper 21.

[0063] A gas containing 41.25 tons per day of ammonia, 17.34 tons perday of carbon dioxide and 3.68 tons per day of water was discharged fromthe top of the scrubbing portion D and was further treated to recoverammonia and carbon dioxide.

[0064] The urea solution containing 1764.23 tons per day of urea, 322.77tons per day of ammonia, 381.63 tons per day of carbon dioxide, 846.97tons of water and 7.24 tons of biuret was discharged from the bottom ofthe stripper 21. This urea solution was further treated to recoverunreacted ammonia and carbon dioxide, and to concentrate the ureasolution to provide 1725 tons per day of urea.

COMPARATIVE EXAMPLE 1

[0065] An example of the production of 1725 tons of urea per day usingan apparatus in which the condenser and synthesis column are separate ispresented below for comparison with Example 1, referring to FIG. 4.

[0066] The condensate from the condenser 216 containing 1216.88 tons perday of urea, 1358.15 tons per day of ammonia feedstock, 1075.69 tons perday of carbon dioxide feedstock, 856.26 tons per day of water and 2.31tons per day of biuret, in which the pressure was raised by an ejector227 driven with 950.13 tons of liquid ammonia from pipe 203 preheated to150° C., was supplied to a urea synthesis column 217 along with 171.36tons per day of carbon dioxide via pipes 13 and 18. The reaction wascarried out at a pressure of 15 MPaG and a temperature of 185° C., witha twenty minute residence time.

[0067] The urea synthesis solution containing 1853.88 tons per day ofurea, 1945.86 tons per day of ammonia, 778.79 tons per day of carbondioxide, 1047.95 tons per day of water and 4.31 tons per day of biuretwas transferred from the top part of the synthesis column 217 to thestripper 221 via pipe 211 and control valve 212 and was contacted andheated with 1040.23 tons per day of carbon dioxide from pipe 219.Unreacted ammonia and carbon dioxide from the urea synthesis solutionwere separated as a mixed gas comprising 1598.58 tons per day ofammonia, 1469.46 tons per day of carbon dioxide and 154.63 tons per dayof water.

[0068] A urea solution containing 1764.22 tons per day of urea, 396.69tons per day of ammonia, 412.76 tons per day of carbon dioxide, 867.45tons per day of water and 7.24 tons per day of biuret was removed fromthe bottom part of the stripper 221 through pipe 225, the ammonia andcarbon dioxide contained therein were separated, and then the ureasolution was concentrated to obtain 1725 tons per day of urea.

[0069] The aforesaid mixed gas from pipe 222 and 25.48 tons per day ofliquid ammonia were supplied to the bottom of the condenser 216, and therecovered solution containing 5.75 tons per day of urea, 452.61 tons perday of ammonia, 545.43 tons per day of carbon dioxide and 341.28 tonsper day of water, which came from separating/recovering unreactedammonia and carbon dioxide was supplied via pipe 208 to the top of thescrubbing portion J in the top part of the condenser 216. Afterabsorbing the uncondensed ammonia and carbon dioxide, the absorbingmedium which came from the bottom part of the scrubbing portion J, wassupplied to the bottom of the condenser 216 through the down-flow-pipe205. The condenser 216 was cooled by means of cooling pipes 206 andmaintained at a temperature of 180° C. and a pressure of 15 MPaG. Theresidence time was twenty minutes.

[0070] The condensate flowed upward from the bottom part of thecondenser, and was collected from the top part by the down-flow-pipe207, the pressure of the condensate raised by ejector 227 andtransferred to the synthesis column 217.

[0071] An unabsorbed mixed gas containing 30.59 tons per day of ammonia,49.45 tons per day of carbon dioxide and 3.88 tons per day of water wasdischarged from pipe 215 of the scrubbing portion J at the top of thecondenser 216, and ammonia and carbon dioxide were subsequentlyrecovered.

[0072] In the process and apparatus of Comparative Example 1, the loadon the stripper is increased (compared to the process and apparatus ofExample 1) due to a decrease in the rate of urea synthesis, and anincrease in the amount of remaining unreacted ammonia and carbon dioxidein the urea solution obtained from the stripper. The amount of gasdischarged from the scrubbing portion of the condensation column alsoincreased.

COMPARATIVE EXAMPLE 2

[0073] An example of the production of 1725 tons per day of urea usinghorizontal condensation and synthesis equipment according to FIG. 5 ispresented below for comparison with Example 1.

[0074] Cooling pipes were provided between one end and the middle partof the horizontal condensation and synthesis equipment 340, to provide acondensation portion K, and the scrubbing portion N for absorbinguncondensed gas was provided above the condensing portion K. Inaddition, a baffle plate 341 was placed in the horizontal condensationand synthesis equipment 340.

[0075] 25.48 tons per day of liquid ammonia were introduced in the areanear the inlet of the cooling pipe 306 in the condensation portion K ofthe horizontal condensation and synthesis equipment 340 through thelines 302 and 303, and 950.13 tons per day of liquid ammonia feedstockpreheated to 75° C. by the pre-heater 320 were introduced into theboundary area between the condensation portion K and the synthesisportion L via pipe 304. Also, a recovered solution containing 5.23 tonsper day of urea, 470.66 tons per day of carbon dioxide, 527.88 tons perday of carbon dioxide and 342.52 tons per day of water, obtained byseparating/recovering the unreacted ammonia and carbon dioxide containedin the urea solution transferred from the stripper, was supplied to thetop part of the above described scrubbing portion N via pipe 308 toabsorb ammonia and carbon dioxide in the uncondensed gas. The absorbingliquid was then allowed to flow down to the cooling portion of thehorizontal condensation and synthesis equipment 340. A mixed gascontaining of 67.64 tons per day of ammonia, 28.02 tons per day ofcarbon dioxide and 5.94 tons per day of water was discharged throughline 315 from the top part of the scrubbing portion N, and ammonia andcarbon dioxide were subsequently recovered. The pressure of thehorizontal condensation and synthesis equipment 340 was maintained at 15MPaG, and the temperature of the condensing portion was maintained at180° C. The condensate obtained in the cooling portion entered thesynthesis portion L (the portion extending from the middle part to theoutlet of the urea synthesis solution) of the horizontal condensationand synthesis equipment 340, where ammonium carbamate was produced fromthe liquid ammonia introduced via pipe 304, the mixed gas (describedlater) introduced from the stripper 321 and the uncondensed ammonia andcarbon dioxide. The heat of reaction maintained the temperature of thesynthesis portion L at 185° C. The residence time in the horizontalcondensation and synthesis equipment 340 was forty minutes.

[0076] The urea synthesis solution containing 1853.90 tons per day ofurea, 1956.40 tons per day of ammonia, 798.37 tons per day of carbondioxide, 1052.96 tons per day of water and 4.31 tons per day of biuret,obtained from the horizontal condensation and synthesis equipment 340,was introduced into the stripper 321 via pipe 311 and control valve 312,and was contacted with 1211.58 tons per day of carbon dioxide introducedvia pipe 313. The mixed gas containing 1628.42 tons per day of ammonia,1656.91 tons per day of carbon dioxide and 160.29 tons per day of waterwas transferred from the top part of the stripper 321 through pipe 322and a gas dispersion unit 342 into the horizontal condensation andsynthesis equipment 340.

[0077] A urea solution containing 1764.23 tons per day of urea, 377.38tons per day of ammonia, 416.24 tons per day of carbon dioxide, 866.79tons per day of water and 7.24 tons per day of biuret was removed fromthe bottom part of the stripper 321 through pipe 325, and unreactedammonia and carbon dioxide therein were separated/recovered to provide1725 tons per day of urea.

[0078] In the process and apparatus of Comparative Example 2, the loadon the stripper increased (compared to the process and apparatus ofExample 1) due to a decrease in the rate of urea synthesis, and becausethe amount of ammonia and carbon dioxide in the urea solution alsoincreased, the load on the low pressure decomposition and recoveringsteps increased. The amount of gas discharged from the scrubbing portionof the condensation and synthesis column also increased.

[0079] By using a vertical condensation and synthesis column with thefunctions of a vertical condenser and a synthesis column combinedtogether, the following advantages can be provided by the processes andapparati of the present invention.

[0080] First, the amount of equipment is reduced, thus simplifying theplacement and operation of the apparati of the present invention.

[0081] Second, since it is not necessary to supply carbon dioxide to thevertical condensation and synthesis column, the full amount of carbondioxide can be supplied to the stripper, compared to conventionalprocesses in which the condenser is separated from the synthesis column.Thus, the rate of stripping increases, resulting in reduction in theamount of ammonia and carbon dioxide in the urea solution obtained fromthe bottom part of the stripper. As a result, the amount of unreactedammonia and carbon dioxide that must be recovered at low pressure isreduced, and the H/C in the vertical condensation and synthesis columnis thus decreased, resulting in an increase in the rate of ureasynthesis.

[0082] Third, since the capacity coefficient of liquid phase (K_(L)a) isabout twice as large as if a horizontal condensation and synthesiscolumn, the rate of synthesis is increased. Also, the amount of gasdischarged from the scrubber in the top part of the verticalcondensation and synthesis column is reduced.

[0083] The priority document of the present application, Japanese patentapplication 202,482/2000 filed Jul. 4, 2000, is incorporated herein byreference.

[0084] Obviously, numerous modifications and variations on the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

What is claimed is:
 1. A urea synthesis process comprising: reactingammonia and carbon dioxide in a vertical condensation and synthesiszone, thereby providing a urea synthesis solution comprising urea,unreacted ammonia, unreacted carbon dioxide and water; transferring theurea synthesis solution from a top portion of the vertical condensationand synthesis zone to a top portion of a stripping zone; introducingcarbon dioxide into a bottom portion of the stripping zone; contactingthe urea synthesis solution with the carbon dioxide in the strippingzone, thereby separating the unreacted ammonia and the unreacted carbondioxide from the urea to provide a mixed gas comprising ammonia, carbondioxide and water; introducing the mixed gas into a bottom portion ofthe vertical condensation and synthesis zone; introducing liquid ammoniainto a bottom portion and a middle portion of the vertical condensationand synthesis zone; contacting and cooling the mixed gas and liquidammonia in the vertical condensation and synthesis zone, therebycondensing a portion of the mixed gas and producing urea; absorbing theuncondensed portion of the mixed gas in an absorbing medium;transferring at least a portion the absorbing medium to a bottom portionof the vertical condensation and synthesis zone.
 2. The urea synthesisprocess of claim 1, wherein the stripping zone has a pressure which issubstantially equal to the urea synthesis pressure.
 3. The ureasynthesis process of claim 1, wherein the urea synthesis solution istransferred to the stripping zone by gravity.
 4. The urea synthesisprocess of claim 1, wherein the stripping zone has a pressure which isequal to or slightly lower than the urea synthesis pressure.
 5. The ureasynthesis process of claim 1, wherein the pressure of the mixed gastransferred to the vertical condensation and synthesis zone is raised by0.05 to 5 MPa.
 6. The urea synthesis process of claim 5, wherein thestripping zone has a pressure which is equal to or slightly lower thanthe urea synthesis pressure.
 7. The urea synthesis process of claim 5,wherein the stripping zone has a pressure which is substantially equalto the urea synthesis pressure.
 8. The urea synthesis process of claim1, further comprising discharging from the stripping zone the ureasynthesis solution from which unreacted ammonia and carbon dioxide hasbeen separated and further treating it to provide urea.
 9. The ureasynthesis process of claim 1, wherein the vertical condensation andsynthesis zone comprises a lower portion extending from the bottom tothe middle portion of the vertical condensation and synthesis zone, andthe liquid in the lower portion is maintained at a temperature of 165 to190° C.
 10. The urea synthesis process of claim 9, wherein the liquid inthe lower portion has a mole ratio of ammonia to carbon dioxide of 2.5to 4.5.
 11. The urea synthesis process of claim 9, wherein the liquid inthe lower portion has a mole ratio of water to carbon dioxide of 1.0 orsmaller.
 12. The urea synthesis process of claim 9, wherein the liquidabove the lower portion is maintained at a temperature of 170 to 200° C.13. The urea synthesis process of claim 9, wherein the liquid above thelower portion has a mole ratio of ammonia to carbon dioxide of 3.5 to5.0.
 14. The urea synthesis process of claim 9, wherein the liquid abovethe lower portion has a mole ratio of water to carbon dioxide of 1.0 orsmaller.
 15. The urea synthesis process of claim 1, wherein the pressurein the vertical condensation and synthesis zone is in the range of 13 to25 MPaG.
 16. The urea synthesis process of claim 1, wherein theresidence time in the vertical condensation and synthesis zone is 20 to70 minutes.
 17. A urea synthesis apparatus, comprising (a) a verticalcondensation and synthesis column comprising a cooler in a portionextending from a bottom part to a middle part of the column; (b) astripper for separating unreacted ammonia and unreacted carbon dioxidefrom a urea synthesis solution comprising urea, unreacted carbondioxide, unreacted ammonia, and water, thereby providing a mixed gascomprising ammonia, carbon dioxide and water; (c) a pipe for supplyingliquid ammonia to a bottom part and a middle part of the verticalcondensation and synthesis column; (d) a pipe having a first open enddisposed in a top part of the vertical condensation and synthesis columnand a second open end connected to a top part of the stripper, wherebythe urea synthesis solution is transferred from the verticalcondensation and synthesis column to the stripper; (e) a pipe forintroducing carbon dioxide into a bottom portion of the stripper; (f) apipe connecting a top portion of the stripper to a bottom portion of thevertical condensation and synthesis column for transferring the mixedgas from the stripper to the vertical condensation and synthesis column;(g) a scrubber disposed in a top part of the vertical condensation andsynthesis column, whereby ammonia and carbon dioxide are absorbed in anabsorbing medium; (h) a pipe connected to the top of the scrubber fordischarging unabsorbed gas; (i) a pipe having a first open end disposedin the scrubber and a second open end connected to a bottom portion ofthe vertical condensation and synthesis column, whereby the absorbingmedium and absorbed ammonia and carbon dioxide are transferred from thescrubber to the bottom portion of the vertical condensation andsynthesis column; and (j) a pipe for discharging an aqueous solutioncomprising urea and unreacted ammonia and unreacted carbon dioxide froma bottom portion of the stripper.
 18. The urea synthesis apparatus ofclaim 17, further comprising pressure raising means disposed in a middleportion of the pipe (d).
 19. The urea synthesis apparatus of claim 18,wherein the pressure raising means is a pump.
 20. The urea synthesisapparatus of claim 17, wherein the bottom portion of the verticalcondensation and synthesis column and the bottom portion of the stripperare placed at substantially the same level.
 21. The urea synthesisapparatus of claim 20, wherein the level is ground level.
 22. The ureasynthesis apparatus of claim 17, further comprising pressure raisingmeans disposed in a middle portion of the pipe (f).
 23. The ureasynthesis apparatus of claim 22, wherein the pressure raising means is ablower.
 24. The urea synthesis apparatus of claim 17, further comprisingat least one means for improving liquid-gas contact in the verticalcondensation and synthesis column.
 25. The urea synthesis apparatus ofclaim 24, wherein the means for improving liquid-gas contact in thevertical condensation and synthesis column is a baffle plate having atleast one opening in a middle portion thereof, thereby allowing gas topass through the baffle plate, and having at least one opening on theperiphery thereof, thereby allowing liquid to pass through.
 26. The ureasynthesis apparatus of claim 25, comprising two or more baffle platesaligned so that the liquid flowing through the at least one peripheralopening flows in a zigzag pattern.
 27. A urea synthesis apparatus,comprising (a) a vertical condensation and synthesis column comprisingcooling means in a portion extending from a bottom part to a middle partof the column; (b) stripping means for separating unreacted ammonia andunreacted carbon dioxide from a urea synthesis solution comprising urea,unreacted carbon dioxide, unreacted ammonia, and water, therebyproviding a mixed gas comprising ammonia, carbon dioxide and water; (c)means for supplying liquid ammonia to a bottom part and a middle part ofthe vertical condensation and synthesis column; (d) means fortransferring the urea synthesis solution from the vertical condensationand synthesis column to the stripping means; (e) means for introducingcarbon dioxide into a bottom portion of the stripping means; (f) meansfor transferring the mixed gas from the stripping means to the verticalcondensation and synthesis column; (g) scrubbing means disposed in a toppart of the vertical condensation and synthesis column, whereby ammoniaand carbon dioxide are absorbed in an absorbing medium; (h) means fordischarging unabsorbed gas from the top part of the scrubbing means; (i)means for transferring the absorbing medium and absorbed ammonia andcarbon dioxide from the scrubbing means to the bottom portion of thevertical condensation and synthesis column; and (j) means fordischarging an aqueous solution comprising urea and unreacted ammoniaand unreacted carbon dioxide from a bottom portion of the strippingmeans.